The present invention relates to an implant for an osteosynthesis device, in particular for the spine, the implant comprising a bone-anchoring device surmounted by a fixing head constituted by two upwardly extending members or lateral branches forming an open U-shaped channel and designed to receive a link rod in order to hold it in place by clamping, via a tapped nut suitable for being screwed onto corresponding threaded portions made on the walls of the lateral branches of the fixing head.
It is generally known to use hooks or pedicular screws as bone-anchoring devices. Numerous devices relate to pedicular screws which have the advantage of providing effective fixing for the implant since a pedicular screw is anchored by being screwed into a pedicle.
Nevertheless, it is not always possible to use pedicular screws, particularly for a vertebra at the end of the spine, or because of regulations in certain countries which allow pedicular screws to be used only under very restrictive circumstances, use thereof being considered difficult and therefore dangerous for the patient.
Thus, in some cases, it is necessary to make use of hooks.
The hooks that are normally used are shaped so as to hook onto a projecting portion of the bones of the spine, such as the foot of a pedicle, or else to fix on the vertebral arch, either on the infralamary side or on the supralamary side.
Nevertheless, it must be observed that using hooks gives rise to numerous drawbacks since a single hook cannot on its own provide effective mechanical fixing as can be done by a screw. Hooks can act effectively only as an abutment when used in association with another hook so as to obtain a clamping effect in which case it becomes almost as effective as a screw.
A hook comprises a hook body having an inner bearing face and an outer fixing face provided with means for fixing to a rod for interconnecting anchoring points. It is the interconnecting rod that serves to hold the hooks on two successive vertebrae.
Such an assembly has the effect of securing two successive vertebrae together by moving them towards each other, but it does not provide any means for acting on either of them separately. In addition, it is necessary to find correct bearing points for both hooks simultaneously, and lateral stabilization is insufficient.
Anchoring devices using hooks have already been proposed that include devices for improving stability. Thus, international patent application WO-A-91/01691 describes a device comprising two facing hooks situated on either side of the vertebra, interconnected by a clamping screw for moving them towards each other on request, each hook having a fixing head in which a rod for linking together and preventing relative movement of the assembly is engaged by clamping nuts on each of said hooks.
That embodiment suffers from the major drawback of requiring clamping action to be taken on two hooks and of relying on the link rod to obtain effective clamping, which amounts to saying that prior thereto, the device remains unstable.
The same clamp system is to be found in documents FR 2,695,550 or FR 2,718,994.
Also known, e.g. from patent FR 2,695,550, is a self-contained implant having its own effective fixing means so that it does not require the link rod whose purpose is to link together two or more successive vertebrae. That patent describes a pedicular clamp having a hook secured to a fixing head constituted by two lateral branches forming an open U-shaped channel and designed to receive a link rod in order to prevent it from moving relative thereto by means of a tapped nut suitable for screwing onto corresponding threaded portions made on the lateral branches of the fixing head. That implant also has a counterhook directed towards the said hook and connected thereto by means of an elongate member, finger or threaded rod. The hook and the counterhook are designed to clamp onto opposite sides of a vetrebra independently of the link rod.
That implant suffers from a major drawback relating to the possibility of relative movement between the hook and the counterhook. It also turns out that the operation of adjusting the spacing between the hook and the counterhook with that kind of implant is not an operation than is simple to carry out successfully.
The invention thus seeks to remedy the above-specified drawbacks by proposing an implant for an osteosynthesis device implementing a hook and a counterhook which are independent of the link rod and which offer the advantages firstly of making it easy to adjust the spacing between the hook and the counterhook, and also to maintain them in a relative position, and secondly of preventing the hook and the counterhook from moving relative to each other in a manner that is safe and effective.
The invention thus provides a solution serving firstly to provide primary stabilization of bone anchoring by means of hooks, by improving the bone anchoring as from the preliminary step of placing the implant on the spine, during the step of reducing deformation of the spine and throughout the subsequent duration of the spine being held in place, and secondly of providing the anchoring with stability that is reliable and permanent so as to avoid loss of reduction in the short or medium term.
To achieve this object, the implant comprises:
a hook secured to a fixing head constituted by two lateral branches in an open U-shaped channel designed to receive a link rod for the purpose of preventing movement relative thereto by means of a tapped nut suitable for being screwed onto corresponding threaded portions made on the lateral branches of the fixing head; and
a counterhook directed towards the hook and freely connected to said hook to enable the hook and the counterhook to clamp onto at least one vertebra, independently of the link rod.
According to the invention, the implant includes:
an elongate member connected at one end to the counterhook and arranged to slide in a bore formed in the top potion of the hook and while lying substantially along a center plane of the hook;
a position control device for holding the hook and the counterhook in position relative to each other; and
an interconnection arrangement between the hook and the counterhook preventing relative motion of the hook and counterhook.
Various other characteristics appear from the following description given with reference to the accompanying drawings which show embodiments and implementations of the invention as non-limiting examples.